Platelet cytoskeleton: immunofluorescence studies on ADP and collagen-activated platelets

Immunofluorescence studies reveal that platelet changes induced by adenosine diphosphate and collagen do not include the reorganization of the cytoskeleton in such a way as to expose actin, alpha-actinin, or vinculin. However, when such platelets were made permeable by saponin, these cytoskeletal proteins were present. In studies with collagen, fluorescence was observed along the fibers at areas of platelet adhesion and where no platelets were seen by phase microscopy. No fluorescence was observed with collagen treated with platelet-poor plasma. Scanning electron microscopy of collagen samples treated with platelet-rich plasma revealed a fibrillar meshwork with single platelets, platelet aggregates, and nodular structures that were smaller in size than individual platelets. These nodular structures may represent remnants of platelets still attached to the collagen after platelet detachment has occurred. These tenacious collagen-platelet membrane-binding sites have associated with them cytoplasmic alpha-actinin and vinculin, proteins that have been proposed by others to anchor actin filaments to the plasma membrane.